1. Field of the Invention
The present invention relate generally to steam turbine rotor blades and, more particularly, to a new turbine blade design having a more aerodynamically efficient profile.
2. Description of the Related Art
Turbine efficiency can be improved by reducing blading losses. Turbine efficiency encompasses several parameters such as steam conditions, cycle arrangement and blading internal efficiency. Of these parameters, internal efficiency is probably the most critical one, since performance and blade efficiency are synonymous.
Two of the major parameters considered in the design of new control stage and reaction stage blading are (1) controlled radial flow distribution to minimize losses and (2) improved aerodynamic performance of stationary and rotating blades.
Control stage blades must operate over a wide range of conditions, such as pressure ratios of 1.2 to 3.5. This is due primarily to the fact that this stage of blading operates from partial arc to full arc of admission and as such the steam velocity leaving the nozzle will be subsonic at full arc of admission to transonic at the primary arc of admission. In the primary arc, the nozzle exit Mach numbers can reach levels of 1.3.
In general, the aspect ratio (height/width) of the control stage blading is small and the flow turning angle across the rotating blade is high, consistent with impulse-type blading. Depending upon the arc of admission, the flow turning angle across the rotating blade can be as high as 140.degree..
Low aspect ratio and high turning angle lead to high secondary flow loss which often can be of the same magnitude as the profile loss, and in many cases may be predominant. The essential goal in improving control stage blading performance, is to minimize the effect of secondary flow, as well as to reduce profile loss.
In one of the areas of turbine blade design, in which the blades of a given row have a twisting profile and thus a constantly changing geometry progressively along the length thereof, it becomes critical to tune the blade so that its resonant frequencies at various vibrational modes fall safely between the harmonics of running speed associated with the turbine so as not to induce destructive vibration.
Other blades have a constant profile, i.e., no twisting along the length thereof. These blades do not require tuning since they tend to be thicker and thus stronger. In particular, when using these blades for rotor blades, they must be strong enough for operation through resonance. However, even with this type of blade, it is desireable to keep the width as small as possible since a small width gives the best performance. If the width is reduced too much, the blade will not be able to withstand load or stress which may cause the blade to fail.
In designing any blade used in a steam turbine, a number of parameters must be scrupulously considered. When designing blades for a new steam turbine, a profile developer is given a certain flow field with which to work. The flow field is determined by the inlet and outlet angles (for steam passing between adjacent rotor blades of a row), gaging, and the velocity ratio, among other things. "Gaging" is the ratio of throat to pitch; "throat" is the straight line distance between the trailing edge of one rotor blade and the suction-side surface of an adjacent blade, and "pitch" is the distance between the trailing edges of adjacent rotor blades. These parameters are well known to persons of ordinary skill in the art and play an important role in the design of every new rotor or stationary blade.
Other general blade design considerations include the following: blades having tenons have to have the location of the blade tenon as close as possible to the center of gravity of the blade; the trailing edge of the blade has to be very close to the edge of the platform; and the center of gravity of the airfoil must be as close to the center gravity of the platform as possible to minimize eccentric stress forces on the root of the blade.
A continuing need exists for blade designs which have increased aerodynamic efficiency, leading to increased thermal efficiency of the turbine, without concomitant losses in structural strength.